Perfect, thanks Allen, now I see the part that I was misreading. I didn't recognize that the only bindings that would be recreated and shadowed in the new environment were those that had explicitly been redeclared in the body, I was thinking `varNames` included the parameter names, which is clearly wrong. So in my examples, the addition of a `var arg;` into the function body, causes the behavior I was referencing. That's an interesting edge case.
Thanks! On Tue, May 9, 2017 at 1:19 PM, Allen Wirfs-Brock <[email protected]> wrote: > You misunderstand. The two scope design is all about preventing closures > from referencing variables declared within the body of the functions. See > https://github.com/tc39/tc39-notes/blob/master/es6/2013- > 09/default-arguments.pdf for the motivation for this design. (but be > aware that there are probably some subtle differences between the final > specified design and what this deck describes. > > In you to test cases, `arg` is not declared within the body of `fn` so > there is no duplicate binding for it. Things would be different if you had > a `var` or `function` declaration for `arg` within the body. > > Allen > > On May 9, 2017, at 12:40 PM, Logan Smyth <[email protected]> wrote: > > Hey all, > We're currently exploring some changes to Babel's behavior for function > parameter default scoping, and I was hoping to validate my understanding of > the spec, because my reading of the spec does not conform to Chrome or FF's > behavior. Alternatively if you know bugs for those engines for this, let me > know. > > My understanding from https://tc39.github.io/ec > ma262/#sec-functiondeclarationinstantiation step #27 is that if function > parameters contain any expressions, the function body is shifted to run in > a declarative environment separately from the params. Per #27.f.i.4.a, the > initial values of the params are copied from from the param environment > into the function body environment, at initial execution time. > > Given that, I'd expect cases such as > ``` > (function fn(arg, setValue = function () { arg = "a string" }) { > setValue(); > > return arg; > })("initial") > ``` > to return `initial`, because the `arg` binding being mutated by `setValue` > is not tied to the `arg` binding that the function returns, as the > `"initial"` value would have been copied into the function body binding > before `setValue` ran. > > Is my understanding there correct? > > Similarly I'd have expected > ``` > (function fn(arg, getValue = function () { return arg; }) { > > arg = "new value"; > > return getValue(); > })("initial") > ``` > > to return `"initial"` because it is returning the binding value from the > declarative environment in the function params and accessed by `getValue` > is not the same environment being mutated by the assignment to `arg1`. > > Is my understanding correct, or is there something I'm misunderstanding? > > Thanks! > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss > > >
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